Dose rate monitor, system and method

ABSTRACT

A radiotherapy dose rate monitor system includes an emitting electrode configured to be impinged by radiotherapy radiation; a collecting electrode configured to form an electrical circuit with said emitting electrode, a current measurement device configured to measure a current through said emitting and collecting electrodes indicative of a dose of said radiotherapy radiation, and a chamber enclosing a gas. Emission of secondary electrons from the emitting electrode provides a majority of the current.

FIELD OF INVENTION

Embodiments of the present invention relate to the field of medicaldevices. More specifically, embodiments of the present invention relateto systems and methods for measuring and monitoring dosage rates oftherapeutic radiation.

BACKGROUND

External beam radiation therapy may be used in the treatment of variouscancers and non-malignant conditions. Generally, ionizing radiation,including, for example, photons, e.g., X-rays, gamma rays, and chargedparticles, e.g., protons and electrons, is directed at an area ofinterest. In many cases, such ionizing radiation is generated by alinear accelerator or a cyclotron.

It is critical to accurately measure the dose of such radiation duringtreatment. For example, radiotherapy is typically very precisely plannedbased on numerous factors, including, for example, tumor type, tumorlocation, and stage, as well as the general health of the patient. Ingeneral, too much radiation may harm a patient, and too little radiationmay not achieve a desired therapeutic effect.

Conventionally, an ionization chamber may be utilized to measureradiation dosage and/or dose rate based on radiation induced ionizationin a gas. A sample gas is enclosed in an ionization chamber between twoelectrodes. The radiation “beam” is directed through the ionizationchamber prior to impacting a patient, causing some of the sample gas tobe ionized. The ionization typically creates a negatively chargedelectron and a positive ion. A voltage applied to the electrodes, forexample 500 volts, collects the electrons on the positive electrode andcollects positive ions on the negative electrode. A current collected bythese electrodes is generally proportional to the radiation dose rate,and may be measured to create a dose monitor. As long as the radiationionizes only a small fraction of the gas, the current will be linearwith respect to dose rate.

FLASH radiotherapy is an emerging radiotherapy regime that appears toreduce radiation-induced toxicities while maintaining a tumor responsesimilar to that of more conventional radiotherapy regimes. FLASHradiotherapy may be characterized as delivering a high radiation rate,e.g., greater than about 40 grays (Gy) per second, that allows for atotal radiotherapy treatment dose, or large fractions of a totalradiation dose, to be delivered in parts of a second, compared toseveral minutes for conventional radiotherapy. For example, aconventional radiotherapy treatment may include a total dose of 12-25grays (Gy) delivered at a rate of up to 0.4 Gy/s, requiring minutes oftreatment time. In contrast, FLASH radiotherapy may deliver a similartotal dose at a rate of 40 Gy/s, requiring a fraction of a second oftreatment time.

However, when radiation dose rates are very high, as is the case withFLASH radiotherapy, conventional dosage monitoring devices become lessaccurate than desired. Due to the high radiation intensity, a great manyelectron/ion pairs are created such that electrons and ions make up asignificant fraction of the sample gas, and ions/electrons fromdifferent tracks encounter each other on their way to the collectingelectrode(s). As a result, recombination between electrons and ionsoccurs at a high rate that varies with the dose rate, and the measuredcurrent no longer corresponds linearly to the radiation dose rate. Thus,conventional dosage monitoring devices are generally not accurate enoughfor use with FLASH radiotherapy.

SUMMARY OF THE INVENTION

Therefore, what is needed are systems and methods for radiotherapy doserate monitoring. What is additionally needed are systems and methods forradiotherapy dose rate monitoring that accurately measure radiotherapydoses of FLASH radiotherapy. Further, there is a need for systems andmethods for radiotherapy dose rate monitoring that accurately measureradiotherapy doses of both conventional radiotherapy and FLASHradiotherapy. There is a still further need for systems and methods forradiotherapy dose rate monitoring that provide external ion chambers fordose verification and quality assurance. There is a yet further need forsystems and methods for radiotherapy dose rate monitoring that arecompatible and complementary with existing systems and methods ofadministering radiotherapy.

In accordance with an embodiment of the present invention, aradiotherapy dose rate monitor system includes an emitting electrodeconfigured to be impinged by radiotherapy radiation; a collectingelectrode configured to form an electrical circuit with said emittingelectrode, a current measurement device configured to measure a currentthrough said emitting and collecting electrodes indicative of a dose ofsaid radiotherapy radiation, and a chamber enclosing a gas. Emission ofsecondary electrons from the emitting electrode provides a majority ofthe current.

According to another embodiment, a radiotherapy dose rate monitor systemincludes a gas enclosed between a complementary pair of electrodes. Thedose rate monitor system is operable to measure current corresponding toa radiation dose rate. The dose rate monitor system is operable in afirst mode or a second mode. In the first mode, which may correspond toa dose rate characteristic of FLASH radiotherapy, less than 20% of thecurrent is due to collection of electron/ion pairs that resulted fromionization of the gas. In the second mode less than 20% of the currentis due to collection of secondary electrons.

According to another embodiment, a radiotherapy dose rate monitor systemincludes a monitor chamber. The monitor chamber includes an uppermounting substrate, a first electrode on an inner surface of the uppermounting substrate, a lower mounting substrate, separated from the uppermounting substrate, a second electrode on an inner surface of the lowermounting substrate, a sidewall coupling the upper and lower mountingsubstrates, and a gas contained within the monitor chamber. The doserate monitor system further includes a voltage source functionallycoupled to the first and second electrodes, and a current sensorconfigured to measure a current through the first and second electrodes.The current is indicative of a radiation dose passing through themonitor chamber. The radiotherapy dose rate monitor system is configuredto measure radiotherapy dose rates of greater than or equal to 40 Gy/sto an accuracy of better than 98%.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthis specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention. Unless otherwise noted, the drawings may not be drawn toscale.

FIG. 1 illustrates a block diagram of an exemplary radiation treatmentsystem that may serve as a platform for embodiments in accordance withthe present invention.

FIG. 2 illustrates a schematic of an exemplary beam path within anexemplary radiation treatment system, in accordance with embodiments ofthe present invention.

FIGS. 3A and 3B illustrate an exemplary monitor chamber, in accordancewith embodiments of the present invention.

FIG. 4 illustrates a schematic diagram of an exemplary pair ofcorresponding electrodes as part of a monitor unit, in accordance withembodiments of the present invention.

FIG. 5 is a simplified flowchart of an exemplary method of measuring aradiotherapy dose rate, in accordance with embodiments of the presentinvention.

FIG. 6 illustrates a block diagram of an exemplary electronic system,which may be used as a platform to implement and/or as a control systemfor embodiments of the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention will be described in conjunction withthese embodiments, it is understood that they are not intended to limitthe invention to these embodiments. On the contrary, the invention isintended to cover alternatives, modifications and equivalents, which maybe included within the spirit and scope of the invention as defined bythe appended claims. Furthermore, in the following detailed descriptionof the invention, numerous specific details are set forth in order toprovide a thorough understanding of the invention. However, it will berecognized by one of ordinary skill in the art that the invention may bepracticed without these specific details. In other instances, well knownmethods, procedures, components, and circuits have not been described indetail as not to unnecessarily obscure aspects of the invention.

Some portions of the detailed descriptions which follow (e.g., method500) are presented in terms of procedures, steps, logic blocks,processing, and other symbolic representations of operations on databits that may be performed on computer memory. These descriptions andrepresentations are the means used by those skilled in the dataprocessing arts to most effectively convey the substance of their workto others skilled in the art. A procedure, computer executed step, logicblock, process, etc., is here, and generally, conceived to be aself-consistent sequence of steps or instructions leading to a desiredresult. The steps are those requiring physical manipulations of physicalquantities. Usually, though not necessarily, these quantities take theform of electrical or magnetic signals capable of being stored,transferred, combined, compared, and otherwise manipulated in a computersystem. It has proven convenient at times, principally for reasons ofcommon usage, to refer to these signals as bits, values, elements,symbols, characters, terms, numbers, data, or the like.

It should be borne in mind, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities. Unlessspecifically stated otherwise as apparent from the followingdiscussions, it is appreciated that throughout the present invention,discussions utilizing terms such as “applying” or “controlling” or“generating” or “testing” or “heating” or “bringing” or “capturing” or“storing” or “reading” or “analyzing” or “resolving” or “accepting” or“selecting” or “determining” or “displaying” or “presenting” or“computing” or “sending” or “receiving” or “reducing” or “detecting” or“setting” or “accessing” or “placing” or “forming” or “mounting” or“removing” or “ceasing” or “stopping” or “coating” or “processing” or“performing” or “adjusting” or “creating” or “executing” or “continuing”or “indexing” or “translating” or “calculating” or “measuring” or“gathering” or “running” or the like, refer to the action and processesof, or under the control of, a computer system, or similar electroniccomputing device, that manipulates and transforms data represented asphysical (electronic) quantities within the computer system's registersand memories into other data similarly represented as physicalquantities within the computer system memories or registers or othersuch information storage, transmission or display devices.

The meaning of “non-transitory computer-readable medium” should beconstrued to exclude only those types of transitory computer-readablemedia which were found to fall outside the scope of patentable subjectmatter under 35 U.S.C. § 101 in In re Nuijten, 500 F.3d 1346, 1356-57(Fed. Cir. 2007). The use of this term is to be understood to removeonly propagating transitory signals per se from the claim scope and doesnot relinquish rights to all standard computer-readable media that arenot only propagating transitory signals per se.

In the following disclosure, exemplary embodiments in accordance withthe present invention are illustrated in terms of a linear acceleratorand radiotherapy photons, e.g., X-rays. However, it will be appreciatedby those skilled in the art that the same or similar principles apply toother systems, including, for example, cyclotrons, and other types ofionizing radiation, including, for example, electrons, protons, and/orother particles. All such systems are well suited to, and are within thescope of embodiments in accordance with the present invention.

In the following descriptions, various elements and/or features ofembodiments in accordance with the present invention are presented inisolation so as to better illustrate such features and as not tounnecessarily obscure aspects of the invention. It is to be appreciated,however, that such features, e.g., as disclosed with respect to a firstdrawing, may be combined with other features disclosed in other drawingsin a variety of combinations. All such embodiments are anticipated andconsidered, and may represent embodiments in accordance with the presentinvention.

Dose Rate Monitor, System and Method

FIG. 1 illustrates a block diagram of an exemplary radiation treatmentsystem 100 that may serve as a platform for embodiments in accordancewith the present invention. Radiation treatment system 100 may besimilar to a TrueBeam® radiotherapy system, commercially available fromVarian Medical Systems, Palo Alto, Calif.

Stand 10 supports a rotatable gantry 20 with a treatment head 30. Thetreatment head 30 may extend into the gantry 20. In proximity to stand10 there is arranged a control unit (not shown) which includes controlcircuitry for controlling the different modes of operation of the system100.

Radiation treatment system 100 comprises a linear accelerator 40, forexample, within gantry 20, utilized to create a radiation beam.Typically, radiation treatment system 100 is capable of generatingeither an electron (particle) beam or an x-ray (photon) beam for use inthe radiotherapy treatment of patients on a treatment couch 35. Otherradiation treatment systems are capable of generating heavy ionparticles such as protons. For purposes of the following disclosure,only x-ray irradiation will be discussed.

A high voltage source is provided within the stand and/or in the gantryto supply voltage to an electron gun (not shown) positioned on anaccelerator guide located in the gantry 20. Electrons are emitted fromthe electron gun into the accelerator 40 where they are accelerated. Asource supplies radio frequency (microwave) power for the generation ofan electric field within the waveguide. The electrons emitted from theelectron gun are accelerated in the waveguide by the electric field, andexit the waveguide as a high-energy electron beam 45, for example, atmegavoltage energies. The electron beam 45 then strikes a suitable metaltarget 50, emitting high energy x-rays 55 in the direction of a patientP.

As illustrated in FIG. 1 , a patient P is shown lying on the treatmentcouch 35. X-rays formed as described above are emitted from the targetin the treatment head 30 in a divergent beam 104. Typically, a patientplane 116, is positioned about one meter from the x-ray source ortarget, and the axis of the gantry 20 is located on the plane 116, suchthat the distance between the target and the isocenter 178 remainsconstant when the gantry 20 is rotated. The isocenter 178 is at theintersection between the patient plane 116 and the central axis of beam122. A treatment volume to be irradiated is located about the isocenter178.

FIG. 2 illustrates a schematic of an exemplary beam path 200 withinexemplary radiation treatment system 100, in accordance with embodimentsof the present invention. It is appreciated that the illustratedcomponents of beam path 200 are exemplary, and all may not be requiredin some embodiments. Additional components, e.g., a flattening filter(not shown), may also be included in accordance with embodiments of thepresent invention. A radiation beam 204 passes through primarycollimator 210, X and Y jaws 230, and multi-leaf collimator 240. Theprimary collimator may comprise a plurality of selectable collimatorsand/or filters, in some embodiments. The primary collimator, X and Yjaws 230, and the leaves of the multi-leaf collimator (MLC) 240typically comprise an x-ray blocking material, and are positioned in thehead 30 (FIG. 1 ) to define the width of the x-ray beam at the patientplane. Typically, the X and Y jaws 230 are moveable and, when fullyopen, define a maximum beam width at the patient plane 116 (FIG. 1 ).The MLC 330 is positioned at the exit of the head 30, to further shapethe x-ray beam. Exemplary MLCs may use up to 120 individuallycontrollable leaves, for example, thin slices of tungsten, which may bemoved into or out of the x-ray beam under the control of systemsoftware.

In accordance with embodiments of the present invention, a monitorchamber 220 is placed within radiation beam 204. Generally, monitorchamber 220 may be placed between a primary collimator 210 and X and Yjaws 230, although that is not required. Monitor chamber 220 is utilizedto measure the radiation dose delivered by radiation beam 204.

FIGS. 3A and 3B illustrate an exemplary monitor chamber 220, inaccordance with embodiments of the present invention. Monitor chamber220 may be utilized, for example, in radiation treatment system 100, tomeasure a radiation dose and/or dose rate, for example. Monitor chamber220 typically provides closed loop feedback to portions of radiationtreatment system 100 (FIG. 1 ) to control the intensity of beam 204.Monitor chamber 220 typically also provides a record of a treatmentdose. Monitor chamber 220 may further function as part of an emergencyshut off capability if a safe and/or a prescribed level of radiation isexceeded. At least a portion of monitor chamber 220 is positioned withinradiation beam 204, as illustrated in FIG. 2 . FIG. 3A illustrates aside-sectional view of an exemplary monitor chamber 220, in accordancewith embodiments of the present invention.

Monitor chamber 220 may be cylindrical, although that is not required.Monitor chamber 220 is well suited to a wide variety of shapes,including shapes having regular and irregular cross sections. Forexample, monitor chamber 220 may have a square, rectangular, orhexagonal cross section, in some embodiments. As illustrated in FIG. 3A,monitor chamber 220 comprises at least one pair of complementaryelectrodes, for example, electrode 320 and electrode 320 a. Thecomplementary pair(s) of electrodes forms an electrical circuit. In someembodiments, one or more of the complementary pair(s) of electrodes maybe mounted to or on upper mounting substrate 310 and/or lower mountingsubstrate 310 a, coupled together by sidewall(s) 315. In someembodiments, one or more of the complementary pair(s) of electrodes mayform or be mounted to other structures.

Monitor chamber 220 generally encloses a gas. For example, the volume316 enclosed by upper mounting substrate 310, lower mounting substrate310 a, and sidewall 315 comprises a gas 350. In some embodiments, gas350 may be sealed within volume 316, e.g., gas 350 is constrained fromexchanging with an atmosphere outside of volume 316. In someembodiments, volume 316 is not so sealed. It is generally desirable forthe mounting substrates 310, 310 a and associated electrodes to behighly transparent to radiation beam 204. For example, such componentsmay be characterized as being thin and of limited mechanical strength.Such mechanical constrains may limit a range of pressures for gas 350,and/or contribute to a requirement for mounting electrodes onsubstrates. Gas 350 may be below, at, or above ambient atmosphericpressure, in some embodiments.

FIG. 3B illustrates a plan view of an exemplary arrangement of fourelectrodes on the inner surface of upper mounting substrate 310, inaccordance with embodiments of the present invention. Embodiments inaccordance with the present invention are well suited to more or fewerelectrodes, as well as to different shape(s) and orientations ofelectrodes. The electrodes need not be substantially planar shapes, asillustrated in FIG. 3B. For example, in accordance with embodiments ofthe present invention, one or more electrodes may comprise threedimensional structures, including, for example, wires, rods, bars, cups,cones, and/or mesh shapes. It is appreciated that all electrodes are notrequired to be the same shape, in some embodiments.

A plan view of electrode(s) of lower mounting substrate 310 a is notillustrated, but may be a mirror image to that of upper mountingsubstrate 310. As will be further described below, electrodes of theupper mounting substrate 310 are paired with corresponding electrodes ofthe lower mounting substrate 310 a (or elsewhere) to form an electricalcircuit. For example, electrode 320 of upper mounting substrate 310 ispaired with electrode 320 a (FIG. 3A) of lower mounting substrate 310 ato form an electrical circuit.

Exemplary upper mounting substrate 310 and exemplary lower mountingsubstrate 310 a may be configured to be perpendicular to, and incidentto radiation beam 204, although that is not required. For example, oneor more substrates 310, 310 a and/or electrodes 320, 320 a, 325, 325 a,330, 330 a, 335, 335 a (FIG. 3A) may not be perpendicular to radiationbeam 204, in some embodiments. Further, one or more substrates 310, 310a and/or electrodes 320, 320 a, 325, 325 a, 330, 330 a, 335, 335 a (FIG.3A) are not required to be parallel with one another, in someembodiments. For example, sidewall 315 (FIG. 3A) may not beperpendicular to radiation beam 204 and may not be parallel with anotherelectrode. Sidewall 315 may form, or support an electrode. In addition,the collecting electrode is not required to be within the radiation beam204, in some embodiments. In general, radiation beam 204 may beconfigured to pass through at least a portion of volume 316.

Exemplary upper mounting substrate 310 comprises four electrodes: innerelectrode 320, inner electrode 325, outer electrode 330, and outerelectrode 335. The inner electrodes, 320 and 325, may be configured tobe completely within the incident radiation beam 204. The innerelectrodes, 320 and 325, are configured to measure a total dose rate ofthe incident radiation beam 204. Any dose rate difference between innerelectrodes 320 and 325 may reveal a beam 204 angle symmetry error.

The outer electrodes, 330 and 335, may be configured to be partiallywithin the beam 204, e.g., on an edge of beam 204. Any dose ratedifference between outer electrodes 330 and 335 may reveal a beam 204position symmetry error.

FIG. 4 illustrates a schematic diagram of an exemplary pair ofcorresponding complementary electrodes as part of a monitor unit 220, inaccordance with embodiments of the present invention. Monitor unit 220comprises positive electrode 455, which may also be known as or referredto as a collecting electrode, for example, electrode 320 a on lowermounting substrate 310 a (FIG. 3A). Monitor unit 220 also comprisesnegative electrode 465, which may also be known as or referred to as anemitting electrode, for example, electrode 320 on upper mountingsubstrate 310 (FIG. 3A). The terms positive electrode and negativeelectrode refer to the relative electrical potential of the electrodesrelative to one another. Only a single pair of complementary electrodes,e.g., electrodes 320 and 320 a, is illustrated. Other pairs ofcomplementary electrodes, e.g., electrode pair 325 and 325 a, electrodepair 330 and 330 a, and/or electrode pair 335 and 335 a (FIG. 3A), ifpresent, are configured similarly, and function in a similar manner.

A voltage source 450 applies a potential difference+V across thepositive electrode 455 and negative electrode 465 in some embodiments.In some embodiments, a potential difference may be shifted, e.g., −V maybe applied to the negative electrode 465, relative to the positiveelectrode 455, when the positive electrode 455 is at ground potential,for example. In some embodiments, a potential difference may be dividedamong the electrodes, e.g., +V/2 is applied to the positive electrode455, and −V/2 is applied to the negative electrode 465. A voltage splitmay be uneven in some embodiments.

It is appreciated that radiation beam 204 will generally have greaterextent than illustrated. For example, radiation beam 204 may be as wide,or wider, than the electrodes 455, 465, in some embodiments. Radiationbeam 204 passing through gas 350 causes some of gas 350 to ionize,creating positive ions 420 and negative electrons 430. In addition,radiation beam 204 produces secondary electrons 410 via interaction withthe conductive material of negative electrode 465, and producessecondary electrons 440 via interaction with the conductive material ofpositive electrode 455.

Radiotherapy is typically delivered in very short pulses. For example, aconventional radiotherapy system may deliver 360 pulses per second, witheach pulse having a duration of about 4 μs. Each pulse may provide adose of about 1 mGy, for example. Such an exemplary protocol deliversabout 0.4 Gy/s on a time average basis. FLASH radiotherapy may becharacterized as delivering a radiation dose greater than or equal to 40grays (Gy) per second, on a time average basis.

Under the conventional art, the potential difference between electrodesmay be on the order of 500 volts when the electrodes 455, 465 areseparated by a gap d of about 1.0 mm, yielding an electric field ofabout 500,000 volts/m. When such a voltage is applied, an electricalcurrent through a monitor unit, e.g., as measured by current sensor 460,is dominated by the ionization of gas 350. For example, negativeelectrons 430 are collected on the positive or collecting electrode 455,and positive ions 420 are collected on the negative electrode 465. Underthe conventional art, the secondary electrons 410, 440, do notsubstantially contribute to current. For example, secondary electrons410, 440 contribute less than 10% to the current.

At high radiation intensities, e.g., equal to or greater than about 2mGy per 4 μs pulse, a great many electron 430/ion 420 pairs are created,such that electrons 430 and ions 420 make up a significant fraction ofthe sample gas. As a result, recombination between electrons 430 andions 420 occurs at a high rate, and the measured current no longercorresponds linearly to the radiation dose rate. Thus, conventionaldosage monitoring devices are generally not accurate enough for use withhigh intensity and/or FLASH radiotherapy. For example, such conventionaldosage monitoring devices are generally not able to achieve greater thanor equal to 98% accuracy in reporting high intensity and/or FLASHradiotherapy dose rates. Accuracy may be determined in comparison toother well-known dosimeter devices that are typically not used duringtreatment, including, for example, external probes and/or filmdosimeters.

In accordance with embodiments of the present invention, voltage source450 may generate a voltage between electrodes 455, 465 that issufficient to repel secondary electrons emitted from an electrode fromcollecting on an electrode, and low enough that ion pairs recombine inthe gas 350 and do not collect on an electrode.

In accordance with embodiments of the present invention, voltage source450 may generate a voltage of 10-100 volts, for example, 40 volts,across the electrodes 455, 465, separated by a gap d of about 1.0 mm.Such combinations of voltages and separation distances result inelectric field strengths of 10,000 volts/m to 100,000 volts/m. Othervoltage and separation combinations producing such a range of electricfield strengths may be utilized in some embodiments. It is appreciatedthat such a voltage and/or field strength is about an order of magnitudeless than is typically applied under the conventional art for a similarseparation between electrodes. Under such a reduced voltage, inaccordance with embodiments of the present invention, ionization of themonitor chamber fill gas 350 does not contribute significantly to thecollected charge. A low bias+V voltage, for example, around 40 volts fora 1.0 mm gap, for an electric field strength of about 40,000 volts/m,between the electrodes 455 and 465, causes and/or allows the ions 420and electrons 430 to recombine almost immediately. Accordingly, the ions420 and electrons 430 generally do not collect on the electrodes 465and/or 455, resulting in an ion collection efficiency of almost zero.For example, ions 420 and electrons 430 contribute less than half of thecurrent.

In accordance with embodiments of the present invention, the desirablerepulsion of secondary electrons emitted from the electrodes isprimarily a function of the voltage applied between electrodes. Thus,higher voltages promote greater repulsion of secondary electrons fromthe electrodes. In contrast, lower electric fields are beneficial topromoting the desirable fast recombination of ionized gas molecules suchthat electron/ion pairs rapidly recombine and do not contribute tocurrent. Accordingly, in accordance with embodiments of the presentinvention, it may be beneficial to increase a gap dimension betweencomplementary electrode pairs in order to decrease an electric fieldbetween electrodes while maintaining a desirably high electricalpotential between electrodes, for example, increasing a gap size toabout 4 mm for a 40 volt bias voltage.

In accordance with embodiments of the present invention, it may bebeneficial to use a gas 350 in a dose monitoring system 220 having ahigher ionization recombination coefficient and/or a higherelectronegativity than ordinary air, for example, a gas comprising ahigher concentration of oxygen, O₂, than air, including pure oxygen,and/or a gas comprising fluorine, including, for example, sulfurhexafluoride, SF₆. Such a higher ionization recombination coefficientand/or a higher electronegativity contributes to faster recombination ofseparated ions, beneficially further reducing a contribution of positiveions 420 and negative electrons 430 to current.

In accordance with embodiments of the present invention, an electricalcurrent through current sensor 460 is substantially due to theinteraction of the high intensity radiation beam 204 with the conductormaterial of the negative or emitting electrode 465. Radiation beam 204knocks out some secondary electrons (SE) 410 with energy of about 40 eVfrom the negative electrode 465. Thus, a negative potential of about 40volts is sufficient to repel such electrons from the negative electrode465. Due to the negative potential of electrode 465, the emittedelectrons are repelled and do not return to the negative electrode 465,but rather are replaced by current in the electrical circuit. Thiscurrent I, which represents the total released electron charge from thesurface of negative electrode 465, is proportional to beam dose rates.Electrons 440 knocked out from positive electrode 455 will combine withions 420 and/or return to the positive or collecting electrode 455, anddo not make up a significant portion of the current. In electrontherapy, the measured current is related to the electron beam currentand energy. The total electron charge may be used for measuring electronbeam current with well-defined electron beam energy. Thus, the totalsecondary electron charge may be used to measure the delivered dose to apatient at any depth.

Embodiments in accordance with the present invention utilize a muchlower voltage and/or a much lower electric field across electrodes in adose monitor system, and utilize a different mechanism of currentgeneration with such electrodes, in comparison with the conventionalart. For example, current generation under the conventional art isdominated by ionized gas. In contrast, in accordance with embodiments ofthe present invention, current generation is dominated by the emissionof secondary electrons from the negative, or emitting, electrode. Inthis novel manner, embodiments in accordance with the present inventionare able to accurately measure high radiation dose rates, e.g., doserates equal to or greater than about 2 mGy per 4 μs pulse, or about 0.7Gy/s on a time average basis, including dose rates of FLASHradiotherapy. For example, embodiments in accordance with the presentinvention may achieve an accuracy of greater than or equal to 98% forhigh intensity and/or FLASH radiotherapy dose rates.

Referring once again to FIG. 1 , many conventional radiotherapy systemsare designed to rotate around the isocenter 178 of patient P, in orderto distribute a radiation dose over all of the surrounding tissue whiledelivering an entire dose to the target tissue. One potential benefit ofFLASH radiotherapy is that it appears to reduce radiation-induced damageto surrounding tissues while maintaining a tumor response equivalent tothat of more conventional radiotherapy regimes. This benefit of FLASHradiotherapy may reduce the benefit(s) of such rotation. Embodiments inaccordance with the present invention provide systems and methods ofaccurately measuring dose and/or dose rates of FLASH radiotherapy.Accordingly, embodiments in accordance with the present invention mayfacilitate non-rotational FLASH radiotherapy, beneficially reducing thecost, complexity, and room-size requirements of such radiotherapysystems.

FIG. 5 is a simplified flowchart of an exemplary method 500 of measuringa radiotherapy dose rate, in accordance with embodiments of the presentinvention. Method 500 may be performed wholly or partially with acomputer system, e.g., computer system 600 of FIG. 6 .

In 510, a voltage is applied across a complementary pair of electrodes,e.g., electrodes 455, 465 (FIG. 4 ), that are part of a dose ratemeasurement chamber, e.g., dose rate measurement chamber 220 (FIG. 2 ).The radiotherapy radiation may be at dose rate intensities correspondingto FLASH radiotherapy. The voltage may be applied during a pulse ofradiotherapy radiation, in some embodiments. In some embodiments, thevoltage may be changed and/or removed when radiotherapy radiation is notpresent, for example, between radiation pulses.

In accordance with embodiments of the present invention, voltage sourcemay be configured to apply a potential difference between thecomplementary pair of electrodes sufficient to repel secondaryelectrons. In some embodiments, the applied voltage may be between30-100 volts. In some embodiments, the voltage source may be configuredto generate an electric field between the complementary pair ofelectrodes such that electrons and ions of a gas within the dose ratemeasurement chamber that are ionized by the radiotherapy radiationrecombine and do not contribute to current between the complementarypair of electrodes. In some embodiments, the electric field strength maybe between 10,000-100,000 volts/m.

In 520, secondary electrons emitted from the electrodes are collected bythe electrodes. It is appreciated that electrons and ions of a gaswithin the dose rate measurement chamber that are ionized by theradiotherapy radiation recombine and do not collect at the complementarypair of electrodes. In 530, a current due to the secondary electronemission is measured to indicate a dose rate of the radiotherapyradiation.

In optional 540, the indication of dose rate is used as feedback tocontrol a dose rate of FLASH radiotherapy.

FIG. 6 illustrates a block diagram of an exemplary electronic system600, which may be used as a platform to implement and/or as a controlsystem for embodiments of the present invention. Electronic system 600may be a “server” computer system, in some embodiments. Electronicsystem 600 includes an address/data bus 650 for communicatinginformation, a central processor complex 605 functionally coupled withthe bus for processing information and instructions. Bus 650 maycomprise, for example, a Peripheral Component Interconnect Express(PCIe) computer expansion bus, industry standard architecture (ISA),extended ISA (EISA), MicroChannel, Multibus, IEEE 796, IEEE 1196, IEEE1496, PCI, Computer Automated Measurement and Control (CAMAC), MBus,Runway bus, Compute Express Link (CXL), and the like.

Central processor complex 605 may comprise a single processor ormultiple processors, e.g., a multi-core processor, or multiple separateprocessors, in some embodiments. Central processor complex 605 maycomprise various types of well-known processors in any combination,including, for example, digital signal processors (DSP), graphicsprocessors (GPU), complex instruction set (CISC) processors, reducedinstruction set (RISC) processors, and/or very long word instruction set(VLIW) processors. In some embodiments, exemplary central processorcomplex 605 may comprise a finite state machine, for example, realizedin one or more field programmable gate array(s) (FPGA), which mayoperate in conjunction with and/or replace other types of processors tocontrol embodiments in accordance with the present invention.

Electronic system 600 may also include a volatile memory 615 (e.g.,random access memory RAM) coupled with the bus 650 for storinginformation and instructions for the central processor complex 605, anda non-volatile memory 610 (e.g., read only memory ROM) coupled with thebus 650 for storing static information and instructions for theprocessor complex 605. Electronic system 600 also optionally includes achangeable, non-volatile memory 620 (e.g., NOR flash) for storinginformation and instructions for the central processor complex 605 whichcan be updated after the manufacture of system 600. In some embodiments,only one of ROM 610 or Flash 620 may be present.

Also included in electronic system 600 of FIG. 6 is an optional inputdevice 630. Device 630 can communicate information and commandselections to the central processor 600. Input device 630 may be anysuitable device for communicating information and/or commands to theelectronic system 600. For example, input device 630 may take the formof a keyboard, buttons, a joystick, a track ball, an audio transducer,e.g., a microphone, a touch sensitive digitizer panel, eyeball scanner,and/or the like.

Electronic system 600 may comprise a display unit 625. Display unit 625may comprise a liquid crystal display (LCD) device, cathode ray tube(CRT), field emission device (FED, also called flat panel CRT), lightemitting diode (LED), plasma display device, electro-luminescentdisplay, electronic paper, electronic ink (e-ink) or other displaydevice suitable for creating graphic images and/or alphanumericcharacters recognizable to the user. Display unit 625 may have anassociated lighting device, in some embodiments.

Electronic system 600 also optionally includes an expansion interface635 coupled with the bus 650. Expansion interface 635 can implement manywell known standard expansion interfaces, including without limitationthe Secure Digital Card interface, universal serial bus (USB) interface,Compact Flash, Personal Computer (PC) Card interface, CardBus,Peripheral Component Interconnect (PCI) interface, Peripheral ComponentInterconnect Express (PCI Express), mini-PCI interface, IEEE 1394, SmallComputer System Interface (SCSI), Personal Computer Memory CardInternational Association (PCMCIA) interface, Industry StandardArchitecture (ISA) interface, RS-232 interface, and/or the like. In someembodiments of the present invention, expansion interface 635 maycomprise signals substantially compliant with the signals of bus 650.

A wide variety of well-known devices may be attached to electronicsystem 600 via the bus 650 and/or expansion interface 635. Examples ofsuch devices include without limitation rotating magnetic memorydevices, flash memory devices, digital cameras, wireless communicationmodules, digital audio players, and Global Positioning System (GPS)devices.

System 600 also optionally includes a communication port 640.Communication port 640 may be implemented as part of expansion interface635. When implemented as a separate interface, communication port 640may typically be used to exchange information with other devices viacommunication-oriented data transfer protocols. Examples ofcommunication ports include without limitation RS-232 ports, universalasynchronous receiver transmitters (UARTs), USB ports, infrared lighttransceivers, ethernet ports, IEEE 1394, and synchronous ports.

System 600 optionally includes a network interface 660, which mayimplement a wired or wireless network interface. Electronic system 600may comprise additional software and/or hardware features (not shown) insome embodiments.

Various modules of system 600 may access computer readable media, andthe term is known or understood to include removable media, for example,Secure Digital (“SD”) cards, CD and/or DVD ROMs, diskettes and the like,as well as non-removable or internal media, for example, hard drives,solid state drive s (SSD), RAM, ROM, flash, and the like.

Embodiments in accordance with the present invention provide systems andmethods for radiotherapy dose rate monitoring. In addition, embodimentsin accordance with the present invention provide systems and methods forradiotherapy dose rate monitoring that accurately measure radiotherapydoses of FLASH radiotherapy. Further, embodiments in accordance with thepresent invention provide systems and methods for radiotherapy dose ratemonitoring that accurately measure radiotherapy doses of bothconventional radiotherapy and FLASH radiotherapy. Still furtherembodiments in accordance with the present invention provide systems andmethods for radiotherapy dose rate monitoring that provide external ionchambers for dose verification and quality assurance. Yet further,embodiments in accordance with the present invention provide systems andmethods for radiotherapy dose rate monitoring that are compatible andcomplementary with existing systems and methods of administeringradiotherapy.

Although the invention has been shown and described with respect to acertain exemplary embodiment or embodiments, equivalent alterations andmodifications will occur to others skilled in the art upon the readingand understanding of this specification and the annexed drawings. Inparticular regard to the various functions performed by the abovedescribed components (assemblies, devices, etc.) the terms (including areference to a “means”) used to describe such components are intended tocorrespond, unless otherwise indicated, to any component which performsthe specified function of the described component (e.g., that isfunctionally equivalent), even though not structurally equivalent to thedisclosed structure which performs the function in the hereinillustrated exemplary embodiments of the invention. In addition, while aparticular feature of the invention may have been disclosed with respectto only one of several embodiments, such feature may be combined withone or more features of the other embodiments as may be desired andadvantageous for any given or particular application.

Various embodiments of the invention are thus described. While thepresent invention has been described in particular embodiments, itshould be appreciated that the invention should not be construed aslimited by such embodiments, but rather construed according to the belowclaims.

We claim:
 1. A radiotherapy dose rate monitor system comprising: anemitting electrode configured to be impinged by radiotherapy radiation;a collecting electrode configured to form an electrical circuit withsaid emitting electrode; a current measurement device configured tomeasure a current through said emitting and collecting electrodesindicative of a dose of said radiotherapy radiation; a chamber enclosinga gas, and wherein emission of secondary electrons from said emittingelectrode provides a majority of said current.
 2. The radiotherapy doserate monitor system of claim 1, wherein ionization of said gascontributes less than 20 percent of said current.
 3. The radiotherapydose rate monitor system of claim 1, wherein said gas is characterizedas having a greater ionization recombination coefficient than air. 4.The radiotherapy dose rate monitor system of claim 3, wherein said gascomprises oxygen at a higher concentration than that of air.
 5. Theradiotherapy dose rate monitor system of claim 3, wherein said gascomprises fluorine.
 6. The radiotherapy dose rate monitor system ofclaim 1, further comprising: a voltage source coupled between saidemitting and collecting electrodes, wherein said voltage source isconfigured to apply a potential difference between said emitting andcollecting electrodes sufficient to repel said secondary electrons. 7.The radiotherapy dose rate monitor system of claim 1 configured togenerate an electric field between said emitting and collectingelectrodes such that electrons and ions from ionization of said gasrecombine and do not contribute to current between said emitting andcollecting electrodes.
 8. A radiotherapy dose rate monitor systemcomprising: a gas enclosed between a complementary pair of electrodes,wherein said dose rate monitor system is operable to measure currentcorresponding to a radiation dose rate, wherein said dose rate monitorsystem is operable in a first mode or a second mode, wherein in saidfirst mode less than 20 percent of said current is due to collection ofelectron/ion pairs at said electrodes due to ionization of said gas andwherein in said second mode less than 20 percent of said current is dueto collection of secondary electrons.
 9. The radiotherapy dose ratemonitor system of claim 8, wherein said first mode corresponds to a doserate characteristic of FLASH radiotherapy.
 10. The radiotherapy doserate monitor system of claim 8, wherein said second mode corresponds toa dose rate of less than 40 Gy/s.
 11. The radiotherapy dose rate monitorsystem of claim 8, wherein said first mode is configured to place apotential of equal to or less than 100 volts across said complementarypair of electrodes.
 12. The radiotherapy dose rate monitor system ofclaim 11, wherein said first mode is configured to place a potential ofgreater than 10 volts across said complementary pair of electrodes. 13.The radiotherapy dose rate monitor system of claim 8, wherein said firstmode is configured to generate an electric field strength of equal to orless than 100,000 volts/m in a region between said complementary pair ofelectrodes.
 14. The radiotherapy dose rate monitor system of claim 13,wherein said first mode is configured to generate an electric fieldstrength of greater than 10,000 volts/m in said region between saidcomplementary pair of electrodes.
 15. A radiotherapy dose rate monitorsystem comprising: a monitor chamber comprising: a first electrode; asecond electrode; a sidewall coupling said first and second electrodes;a gas contained within said monitor chamber; a voltage sourcefunctionally coupled to said first and second electrodes; a currentsensor configured to measure a current through said first and secondelectrodes, wherein said current is indicative of a radiation dosepassing through said monitor chamber, and wherein said radiotherapy doserate monitor system is configured to measure radiotherapy dose rates ofgreater than or equal to 40 Gy/s to an accuracy of better than 98%during patient treatment.
 16. The radiotherapy dose rate monitor systemof claim 15 wherein said current is primarily due to collection ofsecondary electrons on one of said first and second electrodes.
 17. Theradiotherapy dose rate monitor system of claim 15 wherein said gas ischaracterized as having a greater electronegativity than air.
 18. Theradiotherapy dose rate monitor system of claim 15 wherein said gas ischaracterized as having a greater ionization recombination coefficientthan air.
 19. The radiotherapy dose rate monitor system of claim 15configured to generate an electric field between said first and secondelectrodes such that electrons and ions of said gas ionized by saidradiotherapy radiation recombine and do not contribute to currentbetween said complementary pair of electrodes.
 20. The radiotherapy doserate monitor system of claim 15 wherein said voltage source isconfigured to apply a potential difference between said first and secondelectrodes sufficient to repel said secondary electrons from theemitting electrode.
 21. The radiotherapy dose rate monitor system ofclaim 19 configured to apply said potential difference during a pulse ofsaid radiation dose.
 22. The radiotherapy dose rate monitor system ofclaim 15 wherein said first electrode is mounted on a first mountingsubstrate.